Audio focus: test APIs for focus injection

Add new APIs for testing more audio focus behaviors:
- new methods to request/abandon audio focus on a "fake" focus
  owner for which an arbitrary UID can be injected
- new method to query the fade out duration on focus loss

Bug: 169620099
Test: atest AudioFocusTest#testAudioFocusRequestMediaGainLossWithPlayer
Change-Id: I4be4597a994bde99ed5ef98296a4a8a2d1b02d8a